Fabrication of LiNi0.5 + δMn0.5 − δO2 nanofibers by electrospinning

Polycrystalline tetranary LiNi_0.5 + δMn_0.5 − δO₂ nanofibers have been successfully fabricated by a sol-gel assisted electrospinning technique. The structures and properties of fabricated nanofibers were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA). After heat treatment of the electrospun fibers at a temperature of 800 °C, the LiNi_0.5 + δMn_0.5 − δO₂ phase was found without other trace phases. Multilayered nanoparticles with a grain size of 50 nm or less within a single fiber are notable from TEM. In this study, it was shown that the sol-gel assisted electrospun LiNi_0.5 + δMn_0.5 − δO₂ fibers could be formed with the α-NaFeO₂ type crystal structure at a temperature lower than that in a typical solid-state or sole sol-gel process and possess good thermal stability as high as 800 °C.     

Structural transformation and magnetic properties of CaMn1 − xFexO3 − δ (0.0 ≤ x ≤ 0.5)

We have investigated the structural and magnetic properties of CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). Solid state method is used for the synthesis of these samples. Sintering of these compositions at 1300 °C stabilizes higher ionic radii Fe^+ 3 (0.645 Å) at Mn^+ 4 (0.53 Å) site in CaMn_1 − xFe_xO_3 − δ. Structural transformation from orthorhombic to tetragonal to pseudo cubic crystal system and the increase in lattice parameters have been observed with the substitution of Fe at Mn site in CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). The magnetization data show the transformation of G type of antiferromagnetic arrangement of Mn^+ 4 electrons spins in CaMnO₃ into paramagnetic spin type arrangement with the substitution of Fe at Mn site. The compositions x = 0.05, x = 0.1and x = 0.2 show a small kink ~ 100 K in the magnetization data, which resulted due to the competition between antiferromagnetic and paramagnetic states with the Fe substitution.     

Highly (100)-oriented Ce1 − xFexO2 − δ solid solution films prepared by laser chemical vapor deposition

Ce_1 − xFe_xO_2 − δ solid solution films were prepared on amorphous silica substrates by laser chemical vapor deposition using metal dipivaloylmethanate precursors and a semiconductor InGaAlAs (808 nm in wavelength) laser. X-ray diffraction revealed the formation of single Ce_1 − xFe_xO_2 − δ phase at x ≤ 0.15, while CeO₂ and Fe₂O₃ phases were found for higher Fe content. Highly (100)-oriented Ce_1 − xFe_xO_2 − δ (x = 0.02) films were obtained at laser power, P_L = 50-200 W and deposition temperature, T_dep = 800-1063 K. Lotgering factor (200) was calculated to be above 0.8 for films prepared at P_L = 50-150 W. X-ray photoelectron spectroscopy revealed the presence of Fe³⁺, Ce⁴⁺ and Ce³⁺ on solid solution films. Cross-sectional transmission electron microscope images disclosed a film columnar feather-like structure with a large number of nano-scale interspaces. Deposition rates were 2 or 3 orders of magnitude higher than those reported for conventional metal organic chemical vapor deposition of CeO₂.     

Preparation and properties of La1 − xAgyMnO3 + δ thin epitaxial films

We report here the preparation and properties of La_1 − xAg_yMnO_3 + δ thin epitaxial films. The original two-step preparation procedure was developed. At first, La_1 − xMnO_3 + δ were grown epitaxially by metal-organic chemical vapor deposition on the single-crystal substrates (001) and (110) SrTiO₃, (001) LaAlO₃, (111) and (001) ZrO₂(Y₂O₃). Treatment by the vapor of the metallic silver in the oxygen atmosphere (at 1 bar and 20 bar) was the second step resulting in the selective absorption of silver by La_1 − xMnO_3 + δ phase. The value of y depended on the process conditions and revealed different kinetics of the silver absorption for (001) and (110) orientation of La_1 − xMnO_3 + δ films. The films prepared were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersion X-ray analysis, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, electrical resistivity and magnetoresistance measurements in a four-probe configuration. We have found that metal-insulator transition temperature (T_p) in the series La_1 − xAg_xMnO_3 + δ possessed a maximum of 380 K at x = 0.15. Thus, T_p of La_1 − xAg_xMnO_3 + δ films was significantly higher than ever reported in the literature for the La_1 − xAg_xMnO_3 + δ ceramics. La_1 − xAg_xMnO_3 + δ films demonstrated the important role of the ferromagnetic fluctuations above Curie temperature T_c resulting in the sign change of the resistivity curve temperature slope dR / dT and a significant shift of T_p well above T_c. The maximum of the magnetoresistance on the temperature scale was close to dR / dT maximum. The intrinsic magnetoresistance values as high as 22% at 310 K and 50% at 280 K were measured in the magnetic field of 1 T in the series of La_1 − xAg_yMnO_3 + δ epitaxial films.     

Microstructural characterization of Ce1−xTbxO2−δ (0.60 ≤ x ≤ 0.90) sintered samples

To understand the secondary phase formation in ceria based oxide, the microstructure need to be studied in a wide compositional range. However, in most previous studies, the doping concentration is lower than 50 at.%. In this work, the microstructure of Ce_1−xTb_xO_2−δ sintered samples with Tb concentrations of 0.60 ≤ x ≤ 0.90 were investigated by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Besides the fluorite-structured matrix, a secondary phase with a superstructure formed from a structural modulation of the fluorite structure along [1 1 0] and [0 0 1] directions was observed, whose amount and size reached a maximum at x = 0.90. It has a cubic structure with a lattice constant twice as large as that of the fluorite-structured matrix.     

Properties of superconducting, polycrystalline dysprosium-doped Bi1.6Pb0.5Sr2−xDyxCa1.1Cu2.1O8+δ (0 ≤ x ≤ 0.5)

The structural and superconducting properties of dysprosium (Dy) doped (Bi,Pb)-2212 superconductor have been studied. Dy concentration is varied from x = 0.0 to 0.5 in a general stoichiometry of Bi_1.6Pb_0.5Sr_2−xDy_xCa_1.1Cu_2.1O_8+δ. It is found that the Dy atoms enter into the crystal structure by replacing Sr atoms and induce significant change in lattice parameter, microstructure, hole-concentration and normal state conductivity of the system. The critical temperature (T_C) and critical current density (J_C) at self-field of the Dy-doped samples enhance considerably at optimum doping levels. Maximum T_C of 92.3 K (for x = 0.4) and J_C of 1390 A/cm² at 64 K (for x = 0.2) are observed for doped samples as against 79.4 K and 127 A/cm², respectively, for the pure sample. The results are discussed on the basis of the change in hole-concentration due to Dy-doping at Sr-site of (Bi,Pb)-2212 superconductor.     

Study on the chemical stability of Y-doped BaCeO3 − δ and BaZrO3 − δ films deposited by aerosol deposition

Barium cerate (BaCeO₃) has high proton conductivity but rather poor chemical stability in CO₂-containing atmospheres. Barium zirconate (BaZrO₃), in contrast, is a rather stable material, but exhibits poor sinterability. In the present work, powders of Y-doped BaCeO₃ and BaZrO₃ were synthesized via the solid solution reaction method, and dense ceramic membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ were prepared by the aerosol deposition method at room temperature. Aerosol deposition method is a technique that enables the fabrication of ceramic films at room temperature with a high deposition rate as well as strong adhesion to the substrate. The powders and aerosol-deposited membranes were characterized by X-ray diffraction, particle size analysis, scanning electron microscopy, and X-ray elemental mapping. The chemical stability of powders and aerosol-deposited membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ against water and carbon dioxide has been investigated, and it was found that BaZr_0.85Y_0.15O₃ materials showed a better chemical compatibility.     

Phase transitions in Sr1 + xCo0.8Fe0.2O3 − δ oxides

The phase transitions that take place in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (− 0.2 ≤ x ≤ 0.1) oxides are reported here. Thermogravimetric analysis (TGA) showed that the oxides with − 0.2 ≤ x ≤ 0 were prone to undergo oxygen-vacancy disorder-order phase transitions, while others with x = 0.05, 0.1 had more stable crystal structures during oxygen-desorption processes in nitrogen. These results were further confirmed by high-temperature in-situ X-ray techniques. The changes in activation energies of three typical oxides, Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0, 0.1), used as oxygen-permeable membranes were investigated. The phase transitions in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0) have also been detected in differential scanning calorimetry (DSC) profiles.     

Significant improvement of the oxygen permeation flux of tubular Ba0.5Sr0.5Co0.8Fe0.2O3 − δ membranes covered by a thin La0.6Sr0.4Ti0.3Fe0.7O3 − δ layer

We present a new method to improve the oxygen flux properties and stability of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ tube membrane using a thin layer of La_0.6Sr_0.4Ti_0.3Fe_0.7O_3 − δ as protective coatings. The first relevant result is that the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3 − δ protective layer had an extraordinary positive effect on improving the oxygen permeation flux of the tubular Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ membranes. La_0.6Sr_0.4Ti_0.3Fe_0.7O_3 − δ-coated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ tubular membrane showed the highest oxygen permeability with the flux reaching ~ 3 ml cm⁻² min⁻¹ (oxygen purity > 99%) at 950 °C in static atmospheric pressure through a 1.0 mm thick membrane.     

(0 0 6)-oriented α-Al2O3 films prepared in CO2-H2 atmosphere by laser chemical vapor deposition using a diode laser

(0 0 6)-oriented α-Al₂O₃ films were prepared by laser chemical vapor deposition (LCVD) using aluminum acetylacetonate (Al(acac)₃) in CO₂-H₂ atmosphere. The effects of the CO₂ mole fraction (FCO₂) and laser power (P_L) on the crystal phase, microstructure, and deposition rate (R_dep) were investigated. α- and γ-Al₂O₃ mixture films were prepared at P_L = 90 W (deposition temperature of 818 K), whereas (0 0 6)-oriented single-phase α-Al₂O₃ films were obtained at P_L = 110 W (863 K). The texture coefficient and the grain size of the (0 0 6)-oriented films increased with increasing FCO₂. The orientation of the α-Al₂O₃ films changed from (0 0 6) to (1 0 4) to (0 1 2) with increasing P_L (T_dep). The R_dep of the (0 0 6)-oriented α-Al₂O₃ films increased with increasing FCO₂.     

Thin film growth of boron nitride on α-Al2O3 (0 0 1) substrates by reactive sputtering

Boron nitride thin films were grown on α-Al₂O₃ (0 0 1) substrates by reactive magnetron sputtering. Infrared attenuated total reflection (ATR) spectra of the films gave an intense signal associated with in-plane B-N stretching TO mode of short range ordered structure of BN hexagonal sheets. X-ray diffraction for the film prepared at a low working pressure (ca. 1 × 10⁻³ Torr) gave a diffraction peak at slightly lower angle than that corresponding to crystal plane h-BN (0 0 2). It is notable that crystal thickness calculated from X-ray peak linewidth (45 nm) was close to film thickness (53 nm), revealing well developed sheet stacking along the direction perpendicular to the substrate surface. When the substrates of MgO (0 0 1) and Si (0 0 1) were used, the short-range ordered structure of h-BN sheet was formed but the films gave no X-ray diffraction. The film showed optical band gap of 5.9 eV, being close to that for bulk crystalline h-BN.     

Dielectric properties of Ba3−xKxCaNb2O9−δ (0.5 < x < 1.25) (KBCN) double perovskites

We report synthesis, structure and dielectric properties of double perovskite-type Ba_3−xK_xCaNb₂O_9−δ (x = 0.5, 0.75, 1, 1.25) (KBCN). Powder X-ray diffraction (XRD) confirmed the formation of double perovskite-type structure and lattice constant decreases with increasing K in KBCN. AC impedance study showed a single semicircle over the investigated temperatures and frequencies in dry H₂, H₂ + 3% H₂O, 3% H₂O + N₂, while two semicircles were observed at low temperatures in air, which could be attributed to bulk and grain-boundary contributions. Unlike un-doped BCN, KBCN exhibits negligible grain-boundary and electrode effects to the total electrical properties and is consistent with perovskite-type K-doped BaZrO₃. The bulk dielectric constant and dielectric loss were found to increase with increasing K content in KBCN and also found to change with sintering temperature. Among the samples investigated, Ba_1.75K_1.25CaNb₂O_9−δ sintered at 1100 °C showed the highest dielectric constant of 65 at 10⁶ Hz and dielectric loss of 0.14 at 400 °C in air. Isothermal dielectric constant and electrical conductivity at 1 MHz were found to be independent at elevated temperatures, while vary at low-frequency and low temperatures. Below 700 °C, dielectric constant and dielectric loss decreases with increasing frequency, whereas an opposite trend was observed for the electrical conductivity.     

Improved critical current densities in thick YBa2Cu3O7 − δ multilayer films interspaced with non-superconducting YBa2Cu3Ox nanodots

We have fabricated and studied quasi-multilayered thick YBa₂Cu₃O_7 − δ (YBCO) films composed of several YBCO layers interspaced with quasi-layers of non-superconducting YBa₂Cu₃O_x nanodots, grown by Pulsed Laser Deposition on SrTiO₃ (100) substrates. Magnetization J_c(B) at 77.3 K for these thick films showed significant improvement as compared to pure YBa₂Cu₃O_7 − δ films of same or even smaller thickness. A high J_c(B) in our quite thick films (1 μm to 6 μm) provides a very high total critical current per centimetre of the film width, I_c − w. Critical current as high as 830 A per cm width in self field and 77.3 K was achieved in 5 μm thick quasi-multilayer film with non-superconducting YBa₂Cu₃O_x nanodots. Frequency-dependent susceptibility measurements showed also an increase in the pinning potential. The angular dependence of I_c − w at 86.5 K, in 3 T shows a clear indication of anisotropic pinning centres aligned along the c-direction.     

Oxygen permeation and electrical transport of Gd1−xPrxBaCo2O5 + δ dense membranes

Double-perovskite Gd_1−xPr_xBaCo₂O_5 + δ membranes showed appreciable oxygen permeability at moderate temperatures. The overall oxygen permeation process of GdBaCo₂O_5 + δ was found to be controlled mainly by the bulk diffusion step with the membrane thickness larger than 0.8 mm, and the limitation by oxygen surface exchange came into play at reduced thickness of 0.8 mm. The electrical conductivity measurement showed Gd_1 - xPr_xBaCo₂O_5 + δ samples possessed a semiconducting behavior at a wide temperature range below 300 °C and a metallic behavior at 300-850 °C with a high conductivity of nearly 10³ S cm^− 1.     

A comparative study of Pr substitution at Y and Ba sites in YBa2Cu3O7 − δ

Aimed at identifying the factors which suppress the superconductivity in cuprate perovskites I have investigated the effect of substituting Pr ions at the Y and Ba sites in YBa₂Cu₃O_7 − δ system using structural, transport, iodometric, and photoemission studies. The rate of Tc depression in the case when Pr substitutes the Ba site is much higher than the case when it substitutes the Y. This is explained as being due to a combined effect of the factors such as depletion of itinerant holes due to depletion of the oxygen content, the Pr 4f-O 2p hybridization, shortening of c-axis causing Cooper pair-breaking, and the loss of orthorhombicity.     

YBa2Cu3O7 − δ thin films with enhanced film properties grown on sapphire using Y2O3/CeO2 bi-layer buffer

Highly epitaxial, microcrack-free thin films of YBa₂Cu₃O_7 − δ (YBCO) have been grown by pulsed laser deposition on sapphire substrates with a double buffer of Y₂O₃/CeO₂. The Y₂O₃ layer, which has excellent compatibility with CeO₂ and YBCO, has been found to be beneficial in reducing the surface porosity of YBCO films as well as inhibiting a-axis-oriented epitaxial growth. The reduction in porosity is attributed to the presence of the Y₂O₃ layer which acts as a suitable barrier for the chemical reaction occurring at high deposition temperatures between YBCO and CeO₂. Due to the improvement in film quality and surface morphology, enhancement of the self-field critical current density (J_c, 77.3 K) by as high as 30% was obtained for a 650-nm thick YBCO film deposited on Y₂O₃/CeO₂ bi-layer buffer compared to simultaneously-deposited YBCO film on CeO₂ single-layer buffer only.     

The phase transformation and crystallization kinetics of (1 − x)Li2O–xNa2O–Al2O3–4SiO2 glasses

The phase transformation and crystallization kinetics of (1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses have been studied by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) analysis. The crystallization temperature at the exothermic peak increases from 1171 to 1212 K when the Na₂O content increases from 0 to 0.6 mol. The crystalline phase is composed of spodumene crystallization when the Na₂O content increases from 0 to 0.6 mol. The activation energy of spodumene crystallization decreases from 444.0 ± 22.2 to 284.0 ± 10.8 kJ mol⁻¹ when the Na₂O content increases from 0 to 0.4 mol. Moreover, the activation energy increases from 284.0 ± 10.8 to 446.0 ± 23.2 kJ mol⁻¹ when the Na₂O content increases from 0.4 to 0.6 mol. The crystallization parameters m and n approach 2, indicating that the surface nucleation and two-dimensional growth are dominant in (1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses.     

Crossover of magnetoresistance from positive to negative in La0.5Sr0.5CoO3 − σ/Si heterostructure

The magnetoresistance (MR) properties of a heterostructure fabricated by depositing a La_0.5Sr_0.5CoO_3 − σ film on an n-type Si substrate have been studied. The heterostructure exhibits a good rectifying behavior. A negative MR at T = 210 K and a positive MR at T = 300 K are observed for all bias currents whereas; for temperatures ranging from 240 to 280 K the MR changes from being positive to negative with the increase of the bias current. The observed behavior of the MR effect is discussed in terms of current-induced ferromagnetic spin order.     

δ-Phase to defect fluorite (order-disorder) transition in the R2O3-MO2 (R = Sc, Tm, Lu; M = Zr, Hf) systems

We have studied the δ-phase to defect fluorite F* (order-disorder) transition in the R₄M₃O₁₂ (R = Sc, Tm, Lu; M = Zr, Hf) compounds. The temperature of the δ-F* phase transition in Tm₄Zr₃O₁₂ is ∼1600 °C. The rate of this transition in R₄Zr₃O₁₂ (R = Sc, Tm, Lu) decreases markedly with decreasing difference in ionic radius between the R³⁺ and Zr⁴⁺, leading to stabilization of the δ-phases R₄Zr₃O₁₂ with R = Sc and Lu at high temperatures (∼1600 °C). During slow cooling (5 °C/h), the high-temperature defect fluorites F*-R₂Hf₂O₇ (R = Tm, Lu) decompose reversibly to form the δ-phases R₄Hf₃O₁₂. Some of the materials studied exhibit microdomains formation effects, typical of the fluorite-related oxide compounds in the R₂O₃-MO₂ (M = Ti, Zr, Hf) systems of the heavy rare earths. The high-temperature defect fluorites F*-R₄M₃O₁₂ (R = Tm, Lu; M = Zr, Hf) as a rule contain antiphase microdomains of δ-R₄Zr₃O₁₂. After slow cooling (5 °C/h), such microdomains are large enough for the δ-phase to be detected by X-ray diffraction. The conductivity data for R₄M₃O₁₂ (R = Sc, Tm, Lu; M = Zr, Hf) and Ln₂Hf₂O₇ (Ln = Dy, Lu) prepared by different procedures show that the rhombohedral phases δ-R₄M₃O₁₂ (R = Sc, Tm, Lu; M = Zr, Hf) are poorer conductors than the defect fluorites, with 740 °C conductivity from 10⁻⁶ to 10⁻⁵ S/cm. The conductivity drops with decreasing rare-earth ionic radius and, judging from the E_a values obtained (1.04-1.37 eV), is dominated by oxygen ion transport. The highest conductivity, ∼6 × 10⁻⁴ S/cm at 740 °C, is offered by the rapidly cooled F*-Dy₂Hf₂O₇. In the fluorite homologous series, oxygen ion conductivity decreases in the order defect pyrochlore > defect fluorite > δ-phase.     

Soft chemistry synthesis of MxV2O5 + ε·nH2O (MCo, Ni, Cu, Zn) nanowires

Recently a simple method was used for preparing nanowires M_xV₂O_5 + ε·nH₂O (MMg, Mn). It consists of mixing a boiling solution of vanadium oxide with the corresponding metal nitrate. In the present paper, this method was explored for preparing other nanowires layered M_xV₂O_5 + ε·nH₂O compounds (MTi, Cr, Fe, Co, Ni, Cu, and Zn). The results obtained show that If the precipitates are formed immediately after mixing the two solutions, such in case of titanium, chromium and iron, the structure and particles shape of the products are different from the layered compound M_xV₂O_5 + ε·nH₂O. However, if the precipitates start to appear only after several minutes of stirring, such in case of cobalt, nickel, copper and zinc, the phases obtained are similar to layered nanowires M_xV₂O_5 + ε·nH₂O. The amount of the metal inserted “M” was found to be lower than 25 at.% in all as-prepared samples. Nickel containing sample shows the highest inserted amount.